In U.S. Pat. No. 3,208,866 there is described an improved process for the preparation of high quality titanium dioxide pigments. The described process comprises oxidizing a titanium halide such as titanium tetrachloride, in a vapor phase oxidation reactor, with a high oxygen content oxidizing gas at elevated temperatures and in the presence of small regulated amounts of metal ions such as potassium ion. The metal ions can be charged to the vapor phase oxidation reactor either in their elemental state as a vapor, liquid or solid or in the form of various soluble or insoluble inorganic or organic compounds thereof. Whatever the particular form in which the metal ions are employed, preferably they are introduced into the vapor phase oxidation reactor by adding them to or incorporating them into the oxidizing gas stream being charged to the reactor. However, if desired, it also is disclosed that the metal ions can be charged directly into the reactor and to the mixed or mixing titanium halide and oxidizing gas reactants therein ahead of the actual flame of the reaction.
The use of such metal ions is disclosed as providing in-process titanium dioxide pigment products characterized by improved, more uniform particle sizes and more uniform high bulk densities. According to this patent, this latter property is of particular significance, since during the subsequent dry milling of the pigment product such milling is more readily and efficiently carried out. While these characteristics are disclosed as being highly desirable, one serious disadvantage associated with the use of these metal ions involves the separation of the titanium dioxide pigment product from the gaseous stream issuing from the reactor and in which stream the pigment product is suspended. For reasons not fully understood, the use of these metal ions leads to difficulties in effecting an efficient and economical separation of the suspended titanium dioxide pigment product from the gaseous stream in which they are suspended. Specifically, these difficulties have included a diminished capability to effect the desired separation, rapid and frequent blockage of filtration means used in conjunction with this separation and, as a result, a reduction in overall production capacity. Thus, the providing of means permitting the use of such metal ions while overcoming the above mentioned difficulties associated with such use would represent a significant advancement over the current state of the art in this field.